1. Field of the Invention
This invention relates to an electrocatalytically active electrode material for use in electrochemical cells. More particularly, the invention relates to the use of amorphous carbon as an electrode material in electrochemical cells.
2. Description of the Prior Art
In the form of graphite, carbon has been extensively utilized in the fabrication of electrodes for use in electrochemical cells. Indeed, the two most generally satisfactory electrode materials are graphite and platinum. Among their various properties, both graphite and platinum are excellent electrochemical catalysts. Platinum is typically the electrode material of choice for laboratory applications whereas graphite is utilized for industrial applications because of its low cost relative to platinum.
Carbon films have been produced by a variety of vacuum deposition techniques which include electron beam vacuum evaporation, radio frequency sputtering, radio-frequency plasma decomposition of hydrocarbon gases, direct current glow discharge of predominantly hydrocarbon gases with a small fraction of argon, coaxial pulsed plasma acceleration using methane gas, vacuum arc deposition using a graphite cathode, ion beam deposition with argon and hydrocarbon scission fragment ions, and deposition using pure carbon ion beams. Typically, however, sputtering, electron beam evaporation and plasma deposition are the most convenient techniques for the preparation of these films. When produced by decomposition of a hydrocarbon gas, the carbon films may contain small amounts of hydrogen.
The above-described carbon films are very hard and typically have a Mohs hardness of about 6. In addition, the films are generally substantially transparent to visible and infrared light, are essentially inert chemically, and have a resistivity which can range from about 0.1 to greater than 10.sup.11 ohm-centimeter depending on the precise method of manufacture. The carbon in these films is nongraphitic in character and has been described in the scientific literature as diamond-like or amorphous. For the purposes of this application, all substantially nongraphitic carbon which is produced by vacuum deposition techniques is hereinafter referred to as amorphous carbon.
Various studies which have utilized techniques such as X-ray diffraction, electron microscopy and electron diffraction have demonstrated that the carbon which is produced by vacuum deposition techniques is essentially amorphous in character. Unlike graphitic carbon, which is an excellent conductor of electricity, amorphous carbon is a semiconductor with a relatively high resistivity which decreases with increasing temperature. Finally, amorphous carbon is essentially transparent to visible and infrared light whereas graphitic carbon is not. These properties suggest that the carbon atoms in amorphous carbon are four-coordinate as in diamond rather than three-coordinate as in graphite. However, there is no suggestion in the prior art that either diamond or amorphous carbon would have electrocatalytic properties.
An electrochemical cell, in its most simple form, comprises an electrolyte which is in contact with two or more electrodes. Photoelectrochemical cells represent a special class of electrochemical cells wherein at least one of the electrodes is photoactive.
Photoelectrochemical cells are capable of generating electrical energy from solar radiation and also provide a means for the storage of solar energy. The basic photoelectrochemical cell comprises a photoelectrode, a counterelectrode and a reduction-oxidation or redox couple in an electrolyte. The simplest photoelectrode in the basic cell comprises a semiconductor with the front face illuminated by solar radiation and in direct contact with electrolyte solution which contains the redox couple. The back face of the semiconductor is connected to an insulated wire, and a voltage is generated between the back face contact and the counterelectrode with electrons traveling in an external circuit formed by the wires between the two electrodes; and ions pass through the electrolyte between the two electrodes, completing the electrical circuit. The junction between the electrolyte and the semiconductor photoelectrode is a diode junction which acts much the same as a p-n junction in a solid state solar cell. However, since the junction between the electrolyte and the semiconductor is a property of the interface, its formation does not require the precise diffusion of dopant material into the semiconductor which is usually important in a solid state device. Therefore, the photoelectrochemical cell has substantial differences from conventional solid state photovoltaic cells. These differences lead to important advantages over conventional solid state photovoltaic cells, such as the ability to use a broader range of materials for efficient cell operation, the ability to avoid the constraints of lattice parameter matching between adjacent material layers, which is necessary for nearly all solid state photovoltaic devices, and the ability to use small grain size semiconductor material without any substantive decrease in solar conversion efficiency.
The potential advantages of the photoelectrochemical cell offer the promise of a relatively low cost method for the production and storage of useful energy from solar radiation. Unfortunately, this promise has not yet been fully realized. A major problem with photoelectrochemical cells has been the fact that such cells typically have an extremely short lifetime as a consequence of unwanted corrosion effects which occur at the junction between the photoelectrode and the electrolyte solution.
A number of publications have disclosed various attempts to prevent corrosion of the semiconductor photoelectrode in a photoelectrochemical cell. One approach has been to utilize thin protective metal films, particularly gold and platinum, over the base semiconductor [see T. Skotheim, I. Lundstrom, and J. Prejza, J. Elec. Soc.: Accel. Comm., 1625 (1981)]; however, the films must be thin in order to permit light to pass through to the semiconductor, and it is difficult to produce uniform, impermeable, thin metal layers and corrosion still occurs.
A second approach to stabilize the photoelectrode involves the use of an ultra-thin layer of a wide band gap oxide, typically TiO.sub.2 or SnO.sub.2, over the base semiconductor (see A. J. Nozik, Second International Conference on Photovoltaic Conversion and Storage of Solar Energy, Aug. 8, 1978, Cambridge, England). Films such as TiO.sub.2 are transparent but are also insulating in character and, if deposited with thickness sufficient to protect against corrosion, the photogenerated charge carriers cannot penetrate the insulating layer and thus the insulator layer prevents operation of the cell. SnO.sub.2 layers are also transparent to light, are more corrosion resistant than TiO.sub.2, and can be made conductive by doping. However, SnO.sub.2 has virtually no electrocatalytic activity (the ability to enhance the kinetic exchange between electrons in the conducting layer and the redox reaction in the electrolyte solution). Electrocatalytic activity is quite important in driving the desired redox-couple reaction in the electrolyte. Therefore, unless an electrocatalytically active layer is deposited on the SnO.sub.2 layer, a photoelectrochemical cell, which uses SnO.sub.2 alone as a corrosion protective layer, has an extremely low cell output.
A third approach to prevent photoelectrode corrosion involves coating the base semiconductor with an organic conductor layer [see R. Noufi, O. Tench, and L. F. Warren, J. Elec. Soc., 127, 2310 (1980)]. However, severe problems are encountered in aqueous electrolyte solutions with the organic layers showing poor adhesion and, at best, providing protection for only a few days.
A fourth corrosion protection scheme involves the formation of derivative layers over the base semiconductor which are covalently bonded with the surface layer of the base semiconductor, but photoelectrode stability is maintained for only several days [see J. M. Bolts, A. B. Bocarsky, N. C. Palazzotto, E. J. Walton, N. S. Louis, and N. S. Wrighton, J. Am. Chem. Soc., 101, 1378 (1979)].
The prior art fails to disclose a photoelectrode which is stable for an extended period of time and which produces a high cell output with good efficiency. Accordingly, there is a need for an improved corrosion resistant photoelectrode which has a long lifetime and shows improved photocell output and efficiency in the conversion of electromagnetic radiation to electrical power.
Silicon solar cells having an antireflecting coating of amorphous carbon have been described by Moravec et al., J. Vac. Sci. Technol., 20(3), 1982, pp. 338-340. In addition, B. A. Banks et al. have disclosed the preparation of amorphous carbon films on fused silica, copper and tantalum substrates by sputter techniques using an argon ion beam (NASA Technical Memorandum 82873, prepared for the Meeting of the Greater New York Chapter of the American Vacuum Society, Yorktown Heights, N.Y. June 2, 1982). Further, the use of amorphous carbon for wear-resistant coatings, protective coatings, and antireflective coatings has been disclosed by H. Vora et al., J. Appl. Phys., 52(10), October 1981, pp. 6151-6157. However, the prior art fails to either teach or suggest the use of the amorphous carbon as an electrode material in an electrochemical cell.